In modern dentistry, the use of dental implants represents an expensive, but reliable and aesthetically attractive method of treatment for dealing with gaps.
Implants have clear advantages over the alternative possibilities for tooth replacement such as solid non-implant-supported bridges or removable prostheses; in particular they load the jawbone almost identically to the original tooth. A dental implant is an artificial root generally in the technical embodiment of a screw, which is implanted in the jawbone, when the tooth together with its root has been lost. Through knitting of the implant with the bone, the implant assumes almost the same functions as the original root and also transmits forces into the bone, so that the bone is loaded in tension. Through this loading of the jawbone, the bone metabolism is actively stimulated and supported, so that the jawbone is maintained. The toothless jawbone only covered by the gum, under full dentures, is incorrectly loaded owing to absence of introduction of tensile forces into the bone and additionally pressure with superficial action, and over the years continues to disintegrate, sometimes slowly, and sometimes rapidly. Owing to the progressive bone degradation, the whole bone structure changes, which after some years leads to considerable problems.
Dental implants are therefore inserted in the jawbone, so that once healed they provide better prosthetic care for the patient compared with treatment without implants, and maintain the local bone. Thus, for toothless patients complete dentures are often avoided, as the patient can be supplied either with dental prostheses secured on the implants or combined secured-removable prostheses. In the case of edentulous dental arches, the grinding of healthy teeth for producing conventional (non-implant-supported) bridges can be avoided.
The implantological procedure is very extensive and essentially comprises the following steps:                inserting the implant in the jawbone        healing of the implant in the bone        exposing the implant        taking the impression with special impression materials        production of the dental prosthesis (the superstructure)        trial insertion of the dental prosthesis prepared by the dental technician        inserting the final dental prosthesis (final provision of the implant).        
“Superstructure” means, in the context of the present text, an element that is to be joined or has been joined directly or indirectly to a dental implant and is intended to protrude completely or almost completely into the oral cavity. As a rule the superstructure is a dental prosthesis. A superstructure can for example be a crown, a bridge (or a part of a bridge) or a removable prosthesis (or a part of a removable prosthesis). The term “final” such as in the context “final superstructure” or “final dental prosthesis” denotes, in the context of the present text, an element that is not intended from the outset to be replaced with another element after a certain period of time. In contrast, “temporary element” means, in the context of the present text, an element that is intended from the outset to be replaced at a later time point with another element. The adjective “temporary” is to be understood correspondingly. As a rule a connecting element is provided between implant and superstructure, a so-called abutment. In the context of the present text, an optionally present abutment is not considered part of the superstructure. “Connection” of a temporary superstructure to a dental implant means both positioning of the superstructure directly on the implant (without using an abutment) and positioning of the superstructure on an abutment, which is positioned on the implant (indirect connection).
In the treatment described above, the healing phase as a rule takes between 2 and 6 months. This results in so-called osseointegration of the implant. This means that bone grows on the implant surface and the implant becomes firmly anchored in the bone. This does not, however, result in an optimum architectonic orientation of the bone structure, which is able to absorb and transmit forces. In the treatment described above, on subsequent exposure of the implant and through the subsequent provision of the implants with the prosthetic work (the superstructure) there will be a sudden, one hundred percent transmission of forces, thus a complete transmission of the forces produced during chewing, which occurs without a transition phase, onto the implant not previously loaded by forces and the unloaded surrounding bone structure. The disadvantage of the treatment procedure described above is thus the spontaneously occurring, one hundred percent loading of the implants and surrounding bone structure, which may not correspond to the bone situation around the implant and overloads the bone in some situations.
In order to achieve good osseointegration, the bone and the cells therein take time to grow on the surface of the implant and remodel surrounding bone. Until now it has been assumed that time and the quality of the bone are responsible for the speed of healing and for the success of osseointegration. It is disputed whether physiological forces acting on the implant promote osseointegration. Physiological forces would only load the resultant bone structure to the extent permitted by the healing process that has occurred up to then. The healing time can presumably be shortened by said application of physiological forces and a resultant improvement of the healing process.
The expected length of the healing time, without using said technique of application of physiological forces, is based on recognized classifications of bone density and structure, which give recommendations for particular healing times. Generally they are 6 months for the maxilla and 3 months for the mandible, as they have different bone structures and qualities.
Another important precondition for successful implantation is that the implant possesses a so-called primary stability after being inserted in the bone. This means that, directly after insertion, it should not be loose, but should be anchored absolutely, i.e. to the maximum possible extent, solidly in the bone, which can also be measured using instruments, such as those marketed under the name Periotest®. Interestingly, after initial primary stability, the implant is often somewhat looser after about 14 days, and then becomes firmer again. This can be explained by processes of bone loss and remodeling. The implant is probably very sensitive to the effects of external forces during this period.
In recent years there has been new thinking on the healing times required after insertion of an implant. Mainly following patients' desire not to have to wait for such a long time until completion of treatment, a start was made on working with shorter healing times or completely without healing times and in accordance with the alternative just mentioned, with immediate (directly after implantation or within 24 to 36 hours thereafter) temporary or final provision of the implant and loading.
Studies on immediate loading of implants in comparison with the conventional method are contradictory. Some studies showed that poorer healing was achieved with immediate implantation (M. Lorenzoni, C. Pertl, K. Zhang, W. Wegschneider, Clin. Oral Implants Res., 2003, 14 (3) 273-279); other studies found no difference (P. Quinlan et al., Int. J. Oral Maxillofac Implants 2005, 20 (3), 360-370) or came to the restriction that patients must be selected precisely and the loading forces on the implants should be kept as low as possible (G. Romanos, J. Oral Implantol. 2004, 30 (3), 189-197).
The failures with immediate loading appear to be due to transmission of excessive force, too early, onto the implant-bone structure, which in these cases is just undergoing formation and remodeling. This system can probably tolerate certain physiological forces, and these possibly even have a stimulating effect; however, if they become too large, tolerance is lost and the result is not osseointegration, but connective-tissue invagination of the implant.
The conventional method with comparatively long healing times has the disadvantage that these remodeling processes do not develop slowly and continuously, because at first the jawbone is not loaded for quite a long period (no occlusion with the opposing jaw) and later on experiences a sudden relative full loading by the temporary element or the final provision (superstructure), which can also lead to implant losses.
The method of immediate loading is disadvantageous, because according to current knowledge it poses the risk of leading to overloading of the remodeling bone-implant boundary, which in this early phase of healing can then mean loss of the implant.
Based on these findings, the so-called (also termed “conventional” hereinafter) method of progressive bone loading was described (R. Appleton et al., Clin. Oral Implants Res. 2005, 16 (2), 161-167). During this, the forces must be transmitted slowly to the bone. This increasing transmission of force is called “progressive loading”. First a temporary element (a temporary superstructure) based on plastic is inserted, which at first ensures that the temporary dental prosthesis (the temporary element) does not occlude with its antagonist. In this period the patient is required only to consume liquid or semi-solid food. The temporary element is finally modified so that it is brought closer and closer to the antagonistic tooth in several steps, until it is finally in occlusion with the opposing jaw. During this period the jawbone should adapt to the increasing loading, until finally the final treatment is carried out.
The resultant bone training should bring about slow remodeling processes of the bone architecture, induced by physiological and reduced forces. It is precisely this bone architecture that is important for distributing forces in the bone physiologically, as this is a basic principle of nature and of bone.
The temporary dental prosthesis is prepared in the prior art from conventional dental prosthesis plastics of considerable hardness. Radical-polymerizable acrylates are preferably used for this. These systems are described exhaustively in the literature, for example in EP 270915, in EP 630640, in U.S. Pat. No. 6,063,830, in U.S. Pat. No. 5,548,001, in U.S. Pat. No. 4,617,327, and in EP 0677286.
In addition to the system based on acrylate chemistry, spiro-orthoesters and polycyclic ketal lactones have been proposed for the production of dental materials (U.S. Pat. No. 4,387,215). DE 19506222 describes cationically polymerizable materials based on oxetane and oxacyclobutane derivatives. Furthermore, bicycloaliphatic 2-methylene-1,3-dioxepanes are known from DE 4439485. U.S. Pat. No. 5,665,839 discloses radical-polymerizable oxathiepanes, DE 19612004 describes radical-polymerizable multifunctional vinylcyclopropane derivatives, whereas DE 102004002178 proposes monomers that can be crosslinked by ring-opening metathesis.
A disadvantage of the “conventional” progressive bone loading method described above is the fact that patients may only eat mush, and that because no functional chewing forces develop, there is no controllable reduced application of force compared with a situation with functional full loading. If, however, the patient were to eat food that requires chewing, the chewing forces acting on the implant cannot be controlled.
Efforts have already been made in the prior art to provide temporary superstructures for dental implants, with which the disadvantages associated with the use of the various conventional temporary superstructures can be minimized, in particular also the disadvantages associated with the “conventional” progressive bone loading method. Thus, for example, WO 2008/037753 A2 discloses elastic temporary superstructures for dental implants, which consist of materials with an elastic modulus of less than 300 MPa. The temporary superstructures disclosed in WO 2008/037753 A2 are based on special hardenable starting materials, in particular silicones comprising polyatomic crosslinkable groups or optionally substituted crosslinkable polyethers. The hardenable starting materials proposed in WO 2008/037753 A2 have proved very suitable in practice; however, in practice it is difficult and in some cases even impossible to achieve a sufficient adhesion of the superstructures made from the special hardenable starting materials on the usual dental abutments, such as are used in connection with dental implants. According to WO 2008/037753 A2, therefore specially designed abutments are also used, which have undercuts and make positive locking with complementary temporary superstructures possible. However, the use of abutments without undercuts is preferred in practice, as corresponding undercuts cannot be filled with inelastic materials (such as are used for example following osseointegration when using permanent superstructures). Moreover, in practice a connection between a specific abutment of this kind with undercuts and a temporary superstructure in some cases is not sufficiently firm (when the temporary superstructure does not completely fill the undercut) or else almost irreversible (namely when the temporary superstructure engages firmly in the undercuts). Therefore in dental practice abutments are still typically designed so that they have for example a conical cross-section that tapers towards the chewing surface (occlusal surface). Deviation from this typical abutment design is considered undesirable in dental practice.
Document U.S. Pat. No. 7,798,812 A1 bears the title “Temporary dental prosthesis” and discloses temporary dental prostheses that are flexible under loading and largely absorb externally applied loading. Suitable materials mentioned for these temporary elements are once again silicones, but also: rubber, nylon, polyethylene, copolymers, compomers, elastomers, plastics, Teflon and other biocompatible materials. The superstructures disclosed in U.S. Pat. No. 7,798,812 A1 comprise, along with a prosthesis body, an interface structure, which corresponds in its function to a usual abutment. Prosthesis body and interface structure are either linked together integrally or assembled together reversibly, in particular using screw connections. The teaching of U.S. Pat. No. 7,798,812 A1 does not envisage fitting a prosthesis body on a typical commercially available abutment (as explained above). Because the systems according to U.S. Pat. No. 7,798,812 A1 require the use of specially adapted interface structures instead of commercially available abutments, in some cases their use is felt to be disadvantageous.